1. Field of the Invention
The present invention relates generally to the field of medical ventilators.
2. Description of the Background Art
A medical ventilator is a device used to deliver a gas or gases to a patient. This may include air or oxygen, and may also include a variety of additive medicines or treatments. The gas is supplied to a patient via a gas delivery device such as a mask, a nasal cannula, or a tracheostomy tube. The ventilator may be used in cases where it merely assists a patient's breathing (respiratory insufficiency), or may be used in cases where the ventilator must perform breathing for the patient (respiratory failure or when the patient is under the influence of anesthesia). The ventilator may provide a constant pressure airflow, may provide a cyclic airflow, or may provide other pressure patterns. Therefore, a ventilator may need to be capable of providing a gas or gas mixture at either a steady pressure or may need to be capable of providing an output that follows a predetermined pressure or flow volume profile that corresponds to a predetermined breathing profile. The pressure profile may be a cyclic inhalation/exhalation pattern having a varying gas pressure and flow volume profiles. The pressure profile may need to be varied according to a patient's age, health, medical condition, etc.
FIG. 1 shows a prior art ventilator 100 connected to a gas delivery device 101 in the form of a mask. The prior art ventilator 100 includes a blower 105, and a pressure sensor 108. The blower may be used to achieve a gas supply pressure, and may be an electric motor turning an impeller.
In the prior art, a target gas supply pressure has typically been achieved by controlling the blower motor speed. If the pressure of the gas supply is below a predetermined pressure, the motor speed may be increased, and vice versa.
However, a variable blower speed ventilator 100 of the prior art has drawbacks. In order to keep the ventilator air circuit as light and unobtrusive as possible in order to improve wearability, the prior art ventilator 100 generally employs tubing of a small diameter. As a result, airflow resistance is increased, yielding a lower than expected pressure to the patient.
Airflow resistance is usually measured by a constant-flow pressure drop. This is the drop in pressure (usually expressed in units of centimeters of water, or cm H2O) between the entrance and the exit of the tube under conditions of unvarying flow. The standard ventilator hose is typically about 22 millimeters in diameter and about 6 feet long. This creates a constant-flow pressure drop of less than 1-2 cm H2O for reasonable values of flow (such as a breathing airflow of less than 60 liters per minute). Higher peak airflow values for patients suffering from respiratory insufficiency are possible but are rare.
Recent prior art masks include short lengths of a smaller-diameter tubing (generally about 15 millimeters in diameter and about 6-18 inches long) acting as a strain relief between the bulkier air supply hose and the mask. These smaller diameter tubing segments can add 1-2 cm H2O to the pressure drop.
Clinical guidelines for regulating the air pressure to the patient vary depending on the medical use. A CPAP (continuous positive airway pressure) system may be used for nocturnal treatment of obstructive sleep apnea. In a CPAP system, the patient's airway is partially inflated by the positive pressure in order to aid breathing and sleep. Therefore, it is desirable in a CPAP system to hold the target pressure to a tolerance of about plus or minus 2 cm H2O. The acceptable pressure range for other ventilator modes may extend up to plus or minus 5 cm H2O. Particularly for CPAP ventilators, a more resistive patient airflow circuit (i.e., a mask and hose combined) can cause pressure regulation to the patient to vary outside an acceptable range. This creates a need for a ventilator that can more quickly compensate for rapid changes in the airflow rate.
The increased air circuit resistance may be somewhat compensated for by increasing the blower output. However, if the ventilator circuit resistance becomes too great, a blower may not be able to adjust its speed and output quickly enough to correct for pressure changes. In addition, if the blower is trying to quickly go from one pressure extreme to another, the blower direction may need to be reversed in devices of the prior art. These quick changes in motor speed and direction may cause unacceptable levels of motor heating, may require more electrical power to achieve a desired pressure, may cause much higher levels of motor wear, and still do not provide a satisfactorily fast pressure response.
There remains a need in the art for improved ventilator pressure regulation.